1. Field of the Invention
The present invention relates to a composite material, and more particularly, to a composite material comprising a matrix of an aluminum alloy and a reinforcing material such as short fibers, whisker or particles, and further to an aluminum alloy casting comprising a composite material portion made of a matrix of an aluminum alloy and a reinforcing material such as fibers, whisker or particles and having excellent thermal impact resistivity, strength, friction wear resistance and anti melt-adhesion resistance, and still further to a piston of an internal combustion engine made of such an aluminum alloy casting in which a heavily wearing portion is formed of such a composite material.
2. Description of the Prior Art
It has been widely researched to reinforce aluminum alloys with various reinforcing materials such as fibers, whisker or particles in order to improve the wear resistance of aluminum alloys while utilizing the light weigh merit of aluminum alloys. For example, it has been proposed to reinforce aluminum alloys with ceramic fibers, ceramic whisker, a three dimensional net structure of nickel, and cast iron fibers in Japanese Patent Laying-open Publications 58-9386, 57-70734, 59-218341 and 61-132260, respectively.
Those reinforced aluminum alloy composite materials generally exhibit excellent wear resistance superior to aluminum alloys. Although the above-mentioned aluminum alloy composite materials reinforced with ceramic fibers or ceramic whisker have excellent wear resistance, however, they bear a problem that the mating friction member is heavily worn by the hard ceramic reinforcing elements. On the other hand, although the aluminum alloy composite materials reinforced with a three dimensional net structure of nickel or cast iron fibers bear no such problem of heavy wearing of the mating friction member because these reinforcing materials have relatively low hardness, the composite materials by these reinforcing materials bear a problem that their own wear resistivity is not enough. By these prior art reinforced aluminum alloy composite materials, therefore, it is not possible to increase the wear resistance of the composite material members and also to decrease the friction wearing of the mating members.
As a result of extensive experimental researches with respect to the prior art reinforced aluminum alloy composite materials the inventors of the present application confirmed that, if, these prior art composite materials are used for a frictionally sliding member such as a piston of an internal combustion engine exposed to a relatively high temperature such as above 200.degree. C., the aluminum alloy of the matrix melts and adheres to the frictionally sliding mating member, and causes heavy wearing in both of the mutually sliding members. It was also recognized that such melt-adhesion wearing is avoided when the aluminum alloy matrix is at low temperature because the matrix metal does not come into heavy direct contact with the mating member as it is supported by the reinforcing material elements, but when the matrix metal softens at high temperature, the reinforcing material elements are no longer firmly supported by the matrix metal and the matrix metal in turn comes into heavy direct contact with the mating member.
The phenomenon of melt-adhesion wearing was known, and in the art of reinforced metal composite material it was generally proposed, in order to avoid the melt-adhesion wearing, to increase the amount of reinforcing material such as fibers, whisker or particles. The inventors have conducted various experimental researches to confirm the effects of increasing the amount of reinforcing material on avoiding the melt-adhesion wearing. As a result, it was confirmed that if the amount of reinforcing material is increased, the wearing amount of the mating member inevitably increases, and that, because the matrix material exists in any event in the surface region of the composite material, it is not possible to completely avoid the melt-adhesion wearing.
The inventors of the present application have made various experimental researches on the above-mentioned problems with regard to the reinforced aluminum alloy composite materials, and found that the anti melt-adhesion characteristics of the reinforced aluminum alloy composite materials can be much improved without increasing the wearing amount of the mating member by providing the matrix of the reinforced aluminum alloy composite material with intermetallic compounds of Al and particular metallic elements as finely dispersed therein within a certain range of volumetric percentage.
In prior art researches on the reinforced aluminum alloy composite materials, it was generally understood that the matrix of the composite material should serve as a carrier for transmitting forces between reinforcing elements, and therefore the matrix must have relatively high tenacity. Therefore, a great deal of efforts have been spent to prevent or avoid the eduction of such intermetallic compounds in the matrix that would cause embrittlement of the matrix. However, as a result of various experimental researches conducted by the inventors of the present application, it was confirmed that it is significantly effective for improving the anti melt-adhesion resistance of the reinforced aluminum alloy composite materials at temperature above 200.degree. C. to educt intermetallic compounds in the matrix, and by appropriately selecting the types and amounts of the intermetallic compounds, it is possible to obtain composite materials which have excellent wear resistance and anti melt-adhesion resistance at elevated temperature without increasing the wear amount of the mating members.
As described in the above-mentioned Japanese Patent Laying-open Publications 59-218341 and 61-132260, a composite material having an aluminum alloy matrix and including intermetallic compounds is known. However, in the prior art composite material the intermetallic compounds are formed only in a matrix portion around a net structure of reinforcing material or a mass of reinforcing fibers and not in the cell portions of the net structure or the void portions of the mass of reinforcing fibers. In other words, in the internal region of the composite material the matrix is made of only an aluminum alloy which would cause the melt-adhesion at high temperature such as above 200.degree. C.
Since the intermetallic compounds are generally hard and brittle, it is unavoidable that the composite material becomes brittle as a result of inclusion of such intermetallic compounds. Particulary when such composite materials are subjected to severe heating and cooling cycles, cracks are often formed therein due to thermal fatigue.
The inventors of the present application have made various experimental researches to prevent cracks due to thermal fatigue to occur in the composite materials including intermetallic compounds, and found that it is possible to prevent such cracks to occur in an aluminum alloy casting having a surface portion formed of a composite material even under sever heating and cooling cycles and to improve strength of the composite material portion by controlling the shapes of the intermetallic compounds formed in the composite material portion.
In the piston of an internal combustion engine the top ring groove is apt to be easily abraded, and therefore it has been proposed and practiced to reinforce a portion around the top ring groove of the piston by compositely incorporating various types of reinforcing material therein. For example, in the above-mentioned Japanese Patent Laying-open Publication No. 58-9386 filed by the same assignee as the present application, it is described to compositely reinforce a portion around the top ring groove of a piston with ceramic fibers, and in the above-mentioned Japanese Patent Laying-open Publications Nos. 59-218341 and 61-132260 it is described to reinforce a portion around the top ring groove of a piston with a foamed metallic material and cast iron fibers, respectively.
In the above-described pistons in which a portion around the top ring groove is compositely reinforced with a reinforcing material, the wear resistance of the wall surface of the top ring groove and therefore the durability of the piston can be significantly improved as compared with a piston made of only an aluminum alloy. However, when such a piston is used at a relatively high temperature in order to improve the efficiency of the internal combustion engine, the wear at the wall surface of the top ring groove increases, and when the temperature of the wall surface of the top ring groove rises higher than 250.degree. C., such a piston can only exhibit the same wear resistance as a piston made of only an aluminum alloy.
The inventors of the present application have carried out various kind of experiments using such prior art pistons, and found that if such pistons are used under severe thermal conditions such that the wall surface of the top ring groove is heated above 250 .degree. C., the aluminum alloy of the matrix tends to transfer to the top ring, and causes a melt-adhesion wearing on the lower surface of the top ring groove. It was also found that this melt-adhesive wearing occurs by the mechanism that, although at normal temperature the reinforcing material prevents the matrix alloy in the area close to the outer surface of the composite material portion of the piston to directly contact with the top ring, at an elevated temperature the aluminum alloy softens and the reinforcing material can not be appropriately supported by the matrix, so that the softened aluminum alloy comes into direct contact with the top ring,. Therefore, when such prior art pistons are used, it is required that the temperature around the top ring groove is kept below 220.degree. C. at the highest, preferably below 200.degree. C., and, therefore, this is one of the factors which limit the freedom of design of the pistons, in addition to the limits with regard to positioning the top ring groove, determining the dimensions of parts of the piston, forming the oil channels, etc..
Further the inventors of the present application found that in order to reduce the wear on the wall surface of the top ring groove at an elevated temperature such as 250.degree. C. or more, it is required that the composite material should exhibit high wear resistance itself and ensure low wearing of the mating member, and for these purposes the composite materials should satisfy the following two conditions:
(a) The reinforcing material has a high rigidity enough to support the top ring at an elevated temperature. PA1 (b) The matrix has a high thermal resistance at an elevated temperature enough to support the reinforcing material in an appropriate state with no substantial plastic deformation.
It was understood that these two properties are very important, and that the composite material described in the above-mentioned Japanese Patent Laying-open Publication No. 58-9386 is not sufficient in the condition (b), while the composite material described in the above Japanese Patent Laying-open Publications Nos. 59-218341 and 61-132260 are not sufficient in the condition (a).
The inventors of the present application have further conducted researches on the above problems and found that the pistons in which at least a lower surface portion of the top ring groove is defined by a composite material which comprises a matrix of an aluminum alloy and a reinforcing material of short fibers or whisker, tile aluminum alloy matrix including intermetallic compounds of aluminum and predetermined metal elements as finely dispersed therein, the volumetric ratios of the reinforcing material, the intermetallic compounds and the sum of the reinforcing material and the intermetallic compounds being at predetermined appropriate values, can exhibit highly improved wear resistance and melt-adhesion resistance in the lower surface of the top ring groove, without increasing the wear amount of the top ring, thereby enabling to minimize the weight of and to maximize the efficiency of the internal combustion engine.